According to an image forming device using an electrophotographic printing method, a developing device develops an electrostatic latent image formed on a photoreceptor drum (image carrier). The developing device includes (i) a developing roller, which faces with the photoreceptor drum (image carrier), and (ii) a developer tank containing developer. The photoreceptor drum (image carrier) is rotatable. The electrostatic latent image is formed on the photoreceptor drum (image carrier). The developing roller rotates in order to deliver the developer from the developer tank to the photoreceptor drum, in order to develop the electrostatic latent image formed on the photoreceptor drum.
The density of the image developed by the developing device fluctuates according to various factors, so that it is necessary to adjust the density in order to maintain constant image quality.
Conventionally, in the developing device, a density adjustment is generally carried out as follows: (i) a criterial patch image (test image) is developed to (is formed on) the photoreceptor drum, a transfer belt, or the like, and then density of the patch image is detected, and (ii) γ correction is carried out according to the difference between the density detected and a predetermined reference density. In the γ correction, a developing bias (a bias potential of the developing roller) and a potential charged on the photoreceptor drum (a grid voltage of a charging device) are adjusted. In this case, a conversion table is looked up for finding (setting) a correction amount corresponding to a detected patch image density. The conversion table is prepared beforehand based on experimental data, or the like, and is used to convert the patch image density to the correction amount (hereinafter referred to as a developing density correction amount) of the developing bias, the grid voltage, and the like (that is, to find appropriate developing density correction amount of the developing bias, the grid voltage, and the like according to patch image density).
Moreover, in cases where the developer is a binary developer including the toner and carriers, the carriers are left inside the developer tank, and only the toner is used and consumed for the development. The amount of the toner consumed is replenished to the developer tank by toner supplying means.
In the developing device using the binary developer, in order to maintain the image quality, it is necessary to maintain the concentration of the toner in the developer tank to be an appropriate density. On this account, the developing device using the binary developer is generally arranged such that (i) the magnetic permeability of the developer is measured as an index of the toner concentration, and (ii) when a magnetic permeability detection value (detection signal level) exceeds a reference value for a toner supply judgment, the toner concentration is considered to be less than a predetermined value, then the toner is supplied.
Incidentally, in the above-mentioned density adjustment, when the developing bias is too large or the grid voltage is too small, a cleaning field (potential difference between the photoreceptor drum and the developing roller) becomes too small. The problem here is that an image would be developed such that the image is developed also in a portion where no image should be developed (This problem is called “fogging”). On the contrary, when the developing bias is too small or the grid voltage is too large, the cleaning field becomes too large. The problem here is that the carriers of the developer are transported (dropped) onto the photoreceptor drum, or abnormal electrical discharge (pinhole leak) occurs. In some cases, the carriers transported onto the photoreceptor drum would be rubbed by a cleaning blade. This would possibly cause a damage on the photoreceptor drum. On this account, there is a limit to the developing density correction carried out by adjusting the developing bias and the grid voltage.
Conventionally, for example, Japanese Laid-Open Patent Publication No. 190993/1999 (Tokukaihei 11-190933, published on Jul. 13, 1999) describes means of adjusting the developing density: in cases where an output correction amount (grid correction amount) of a charging apparatus (charging device) is equal to or more than a predetermined value, a toner concentration reference value (which corresponds to the magnetic permeability reference value) is changed accordingly. That is, the above publication discloses such an arrangement that, in cases where a correction width of the grid voltage according to the density of the patch image is equal to or more than a predetermined width, the reference density of the developer (that is, the toner concentration) is increased or decreased accordingly. According to the method in the above publication, it is possible to prevent a fog phenomenon and the transport of the carrier to the drum, and also possible to widen a correction range of the developing density. In this way, it is possible to maintain constant image quality for a long time while coping with various situations.
However, different ideal conversion tables for conversion from the patch image density to the developing density correction amount, that is, different correction references of the developing density are required by different developing density. On this account, the technique described in Japanese Laid-Open Patent Publication No. 190933/1999 faces following problem: in cases where the magnetic permeability reference value is changed from a normal reference value (that is, the magnetic permeability reference value corresponding to the conversion table), it is impossible to set the developing density correction amount appropriately, so that the accuracy of the developing density adjustment is deteriorated.
Meanwhile, instead of adjusting the magnetic permeability reference value according to the output correction amount of the charging apparatus, it is possible to arrange such that the magnetic permeability reference value is adjusted according to the toner concentration, that is, according to the patch image density. However, because the developer tank containing the developer has a fixed capacity, it takes time to attain the uniform toner concentration in the entire developer tank by stirring the developer after the toner supply. On this account, the problem here is that developing density correction performed by adjusting the magnetic permeability reference value according to the patch image density requires a long time each time it is carried out.